Diagnostics: if a line is longer than 4096 characters, don't print it.

Specifically, don't print snippets, caret diagnostics, or ranges for
lines over 4096 characters. We copy the line around a few times in our
diagnostics machinery, and we have to print a caret line that's just as
long. This uses a lot of memory just to create a poor user experience as
we print out a line much too long for anyone to read...or spend extra
energy trying to fit it to -fmessage-length.

<rdar://problem/13106850>